Background: This is the most common retinal pathology in patients who are HIV positive, often manifesting as cotton-wool spots.

Pathophysiology: The cause of retinal microvasculopathy in patients who are infected with HIV is similar to those suggested for conjunctival vascular changes. The specific etiology of the microvascular changes has not been elucidated completely; however, increased plasma viscosity, immune-complex deposition, and a direct cytopathic effect of the virus on the retinal vascular endothelium are believed to be involved. It has been suggested that the HIV infection of the retinal vascular endothelium alone is not sufficient to account for the arteriolar occlusion responsible for cotton-wool spots. The arteriolar occlusion in HIV microvasculopathy leads to interruption of the axoplasmic flow, which manifests as cotton-wool spots.

Frequency: HIV retinal microvasculopathy occurs in as many as 50-70% of patients who are HIV positive. However, it is likely that the increased use of the newly available, highly active antiretroviral agents has lowered the prevalence of the retinal microvasculopathy seen in these patients

Cotton-wool spots are fluffy white, superficial retinal lesions with irregular borders, located in the posterior pole. Their size may vary from 1/10th to 1/2 disc diameter and their number from one single lesion to ten or more. They may disappear spontaneously in the course of a few weeks but are often replaced by new ones in other places. They are clinically indistinguishable from similar lesions observed in diabetes, anaemia, leukaemia or collagen disease. On pathological examination, cotton-wool spots consist of small areas of thickening and oedema of the retinal nerve fibre layer and indicate infarcts in the nerve fibre layer. Cotton-wool spots correspond to the closure of a precapillary arteriole.

Cotton-wool spots are present in 25 to 50% of adult AIDS patients, but are notably absent in HIV-infected children. Although they are occasionally seen in the early stages of HIV infection, it is typically a manifestation of more advanced disease and so the presence of HIV-related retinopathy is a poor prognostic sign.Cotton-wool spots do not cause clinically significant visual symptoms. In some instances the differential diagnosis with incipient CMV retinitis may be difficult.

from the above articles it is clear that the ans is not CMV.there is doubt between immune reax and direct virus invasion but as the role of immune complexes in causing cotton wool spots has not yet been proven the best ans is- DIRECT VIRUS INVASION OF THE CAPILLARIES.